I work in utility scale solar and have recently been introduced to the concept of bi-polar solar sites. Can someone please give me a basic overview of what they are, how they work, common issues, and possible ground fault solutions?
thanks!
A bipolar solar array operates at two DC voltages that are symmetric relative to ground. You build two otherwise-identical arrays that source each inverter. One of those arrays (called the "negative array") is positively grounded, the other array (called the "positive array") is negatively grounded. They each have to have the same number of modules in series, and the same number of strings. Depending on your design, you might route both pairs of full conductors for each array polarity back to the inverter, or you might remotely tie the grounded conductors together and only pass a reference wire back to the inverter to reference the voltage at ground.
Suppose you color code as follows:
Negative array ungrounded - Black
Negative array grounded - Gray
Positive array ungrounded - Red
Positive array grounded - White
And suppose the system is nominally 600V to ground.
The red wire is +600V to ground
The black wire is -600V to ground
The red wire is 1200V relative to the black wire
The gray wire is at ground voltage, and carries the return current of the black wire
The white wire is at ground voltage, and carries the return current of the red wire
If you remotely tie the white and gray wires, the two arrays operate in series, with this remote tie referenced to ground. Negligible current travels on the remote tie reference wire. The actual current that the inverter sees, travels to it on the red wire, and returns to the arrays on the black wire. If these two conductors are routed together, you'd need 1200V worth of insulation, which you'd do with 1kV/2kV PV wire.
The reason bipolar systems exist, is that it allows the DC side of the array to source both halves of the inverter's AC waveform. So it significantly reduces the amount of circuitry needed inside the inverter, and means a more economical inverter for any given KW rating.
